A membrane filter plate of this kind is known from DE-OS No. 19 60 821. The base member is here made of a frame in which a cup-shaped plate is welded. Its edge forms with a step-form recess in the edge of the base member a swallow-tail groove which serves for securing the membrane. The membrane is made cup-shaped and has a border for clamping which on its outer edge carries a swallow-tail form rib directed toward the base member and pressed into the swallow-tail groove. The membrane is thereby held securely in place even when the filter plate press is opened and the pressed filter cake is removed. However, with this filter membrane plate, the membrane does not have as long a useful life as is known for other membranes of the same elastomeric material.
It has been supposed that the shortening of the useful life of the diaphragm is attributable to the stress applied to the material through the high pressure to which it is subjected in the filter plate press. To increase the useful life of the membrane, there is proposed in DE-0S No. 23 22 044 a membrane filter plate designed to work with lower pressure. With this, the plane edge of the base member has a swallow-tail form groove in which is received a swallow-tail form rib provided on the outer edge of the membrane. Two membranes lie on the adjacent base member. The edge planes of the base member in the pressed condition of the membrane filter plates, a spacing which corresponds to twice the edge thickness of one membrane. The force of the filter plate press is here transmitted only by the membrane edge. With lower filter pressure, this is a practical possibility. However, with higher filter pressure, the pressure of the filter plate press must be raised in order to obtain the required sealing. The pressure transmission from the membrane edge to membrane edge leads to a flow of the elastomeric material of the membrane whereby the useful life of the membrane is further shortened.
The invention has recognized that the shortened useful life of these known membranes is attributable to the form of the membrane. In order to press the remaining fluid from the filter material at the end of a filter operation, the membrane is acted upon by compressed air which is introduced between the base member and the membrane. This compressed air compacts the filter cake. Hereby the membrane is subjected to a surface area reduction which is attainable in a cup-shaped membrane only through folding. However, folding extraordinarily shortens the useful life of an elastomeric work piece.
With both of the known membrane filter plates, the cup-shaped construction is required in order to obtain a satisfactory fastening of the membrane to the base member. In the filter presses in which these membrane filter plates comprise many members installed parallel to one another, the slurry, that is the yet unfiltered suspension of solid particles in a fluid, is introduced under considerable pressure (10 bar) into the cavities between filter cloths stretched between two adjacent membrane through panels running inside the membrane filter plates which connect individual cavities with one another. Thereby the slurry must not enter into the cavities between the filter cloths and the membranes or the cavities between the membranes and the base members.
In EP-0S No. 0030905, introduction of the slurry into the membrane filter plates is effected through an opening in the membrane filter plate and a device installed in this opening to secure membranes and filter cloths on both sides of a base member, such device comprising two pieces of tubing provided with flanges which are inserted through an opening in the base member, the membranes and the filter cloths and connected with one another and the flanges of which press the filter cloths and the membranes against the filter plate. These pieces of tubing are pressed of sheet metal. With one of them, the free end of one piece of tubing is bent over for the fastening operation. Such manner of fastening is only possible where the filter cloths are either generally not, or only rarely changed. For a changing of the filter cloths, it is necessary that the piece of tubing bent over to form a flange must be so bent that it cannot be used again. Moreover, the other piece of tubing is usually also damaged. The required production of the pieces of tubing out of sheet metal limits the use of the filter plate thus agressive slurries cannot be filtered. The filter cloths let the filtrate through but hold back the solid particles in the space between two filter cloths. The filtrate is thereupon conducted out of the space between the filter cloths and the membranes through channels into a filtrate discharge channel which penetrates all of the membrane filter plates installed in the filter press.